Ultrasound dating birth
The femur (thigh bone) length seems to hold on to it’s accuracy longer than the other parameters, but after 36 weeks it isn’t foolproof.
Two ultrasounds taken one month apart that agree with each other on when the due date is, yield a very accurate and reliable answer.
Even when the last period is known, ultrasound is reassuring to demonstrate adequate growth, especially when there’s a risk of delayed growth, as in hypertension or smoking, or if there’s the risk of exaggerated growth, as in gestational diabetes.
It is not uncommon for babies that are labeled “Large for Gestational Age (LGA)” and “Intra Uterine Growth Restriction (IUGR)” to have monthly or even weekly ultrasounds during the pregnancy.
The software has certain measurement scales based on data from large populations, and your baby’s measurements are put into this scale.
Before twelve weeks, it’s difficult to measure much more. In the second trimester, (past the first 12 weeks), the other measurements can be added.Most babies, except in cases of early IUGR and deformities, grow about the same until 20 weeks or so.After that, the variations among babies’ measurements cause increasing inaccuracies.Ultrasound has become so helpful that obstetricians now refer to the time before it was used routinely as “the olden days.” We use it to diagnose twins early on; we use it to document appropriate growth as the pregnancy progresses; we use it to determine fetal health; and we use it to guide conversion of breech to vertex (head-first) position and to guide amniocentesis.Of all of these uses, dating the pregnancy is the most common reason to use ultrasound, particularly when the expectant mother cannot remember the date of her last period (as in breast-feeding or irregular cycles).